Mycobacterium tuberculosis is a leading cause of death by infectious disease in the third world and is sharply increasing in the United States, primarily because of the AIDS epidemic. Strains of M. tuberculosis have emerged which are resistant to the many anti-tuberculosis drugs, including isoniazid and rifampicin. Cubist Pharmaceuticals is targeting the aminoacyl-tRNA synthetases, an essential family of enzymes involved in protein synthesis, to develop a novel class of antibiotics. The tRNA synthetases have significantly diverged through evolution, leading to distinct sequence and structural variabilities which facilitate rational drug design for species-specific inhibition. The goal of the research proposed is to develop drug-screening systems for cloned and expressed M. tuberculosis tRNA synthetase genes via enzyme and whole cell assays. The M. tuberculosis genes will be expressed in bacterial and/or yeast null strains which lack the appropriate tRNA synthetase. These expression systems will serve as convenient tester strains to screen small molecules for potential drug candidates. Effective inhibitors can be subsequently screened for specificity against a parallel tester strain for human tRNA synthetases. Discrimination of human and M. tuberculosis tRNA synthetases will yield a powerful drug candidate for the treatment and control of TB.